Thermotropic liquid crystalline polymers are generally wholly aromatic condensation polymers that have relatively rigid and linear polymer chains so that they melt to form a liquid crystalline phase. A typical process for producing liquid crystalline aromatic polyesters involves mixing one or aromatic diols and dicarboxylic acids and/or hydroxycarboxylic acids with enough of a carboxylic acid anhydride (e.g., acetic anhydride) to acetylate the hydroxyl groups of the diols and/or hydroxycarboxylic acids present. Once formed, the acetylated monomers are thereafter heated to a high temperature to initiate a condensation reaction in which the monomers are converted to a polymer. To favor a reaction equilibrium that optimizes the production of a high molecular weight polymer, byproducts of the condensation reaction (e.g., acetic acid, phenolic derivatives, etc.) are generally removed. The mixture is eventually heated to a relatively high temperature, typically in latter stages under vacuum, to produce the final liquid crystalline polymer. This is done while the process mixture is a liquid (in the melt).
To form parts from such liquid crystalline polymers, “injection molding” techniques are typically employed in which polymer pellets are injected into a heated mold cavity to form a mass that is thereafter cooled and hardened so that it assumes the shape of the cavity. The most common technique for injecting the polymer into the mold cavity involves the extrusion of the polymer through a single- or twin-screw extruder. Unfortunately, however, the high shear rates associated with such extrusion processes can cause the polymer to undergo extensive shear orientation, which leads to a part having anisotropic mechanical properties. This is particularly problematic when attempting to form “stock shapes”, such as rods, plates, bars, etc., which typically require further machining that can damage parts of an anisotropic nature.
As such, a need exists for an improved technique of forming parts from liquid crystalline polymers, particularly those that may possess substantially isotropic mechanical properties.